Magnetic Tunnel Junctions with a Nearly Zero Moment Manganese

Dec 6, 2018 - A magnetic nanolayer with a perpendicular magnetic easy axis and negligible magnetization is demonstrated. Even though a manganese ...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Magnetic Tunnel Junctions with a Nearly Zero Moment Manganese Nanolayer with Perpendicular Magnetic Anisotropy Kazuya Z. Suzuki,*,†,‡ Shojiro Kimura,§ Hitoshi Kubota,⊥ and Shigemi Mizukami*,†,‡,∥ †

WPI Advanced Institute for Materials Research, ‡Center for Spintronics Research Network, §Institute for Materials Research, and Center for Science and Innovation in Spintronics (Core Research Cluster), Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan ⊥ National Institute of Advanced Industrial Science and Technology (AIST), Spintronics Research Center, Tsukuba 305-8568, Japan Downloaded via YORK UNIV on December 9, 2018 at 00:56:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: A magnetic nanolayer with a perpendicular magnetic easy axis and negligible magnetization is demonstrated. Even though a manganese metal is antiferromagnetic in bulk form, a few manganese monolayers grown on a paramagnetic ordered alloy template and capped by an oxide layer exhibit a strong perpendicular magnetic anisotropy field exceeding 19 T as well as a negligible magnetization of 25 kA/m. The nanolayer shows tunnel magnetoresistance. Moreover, the perpendicular magnetic anisotropy for the nanolayer can be reduced by applying an electric voltage. These findings will provide new insight into a creation of new nanolayer magnets. KEYWORDS: magnetic tunnel junction, tunnel magnetoresistance, ferrimagnets, perpendicular anisotropy, voltage control

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properties only in atomically well-ordered crystalline states. A technique for creating MTJs comprising a nanolayer of these alloys thin enough to be switched by electrical means still is being developed, even though the template technique using a CoGa ordered alloy was recently found to be powerful.17 Therefore, alternative nanolayer materials having these extraordinary properties must be found. In this letter, a novel magnetic nanolayer is reported. Bulk Mn is an antiferromagnetic metal in an ambient environment, whereas a few monolayers of Mn deposited on the (001) surface of a CoGa paramagnetic ordered alloy template and capped by a MgO barrier show finite magnetizations as well as TMR effect. Furthermore, the Mn nanolayer exhibits a large PMA as well as VCMA. Currently, no other nanolayers are known to possess these extraordinary properties. Figure 1a displays the junction resistance R, measured as a function of the out-of-plane and in-plane magnetic field H for an applied bias voltage V of −50 mV, for the MTJs of Cr (40)/ paramagnetic CoGa (30)/Mn (0.7)/MgO (2.4)/CoFeB (3)/ Ta (3)/Ru (5) (thickness is in nm) grown on a (100)-MgO single crystalline substrate. In the MTJs, Cr/CoGa, CoFeB, and Ta/Ru serve as underlayers, a counter-magnetic electrode, and capping layers, respectively. Hereafter, the positive bias voltage is defined with respect to the CoFeB electrode. The out-of-plane R−H curve exhibits a well-defined hysteresis, and the in-plane curves show the gradual change with no hysteresis.

agnetic tunnel junctions (MTJs) comprise an insulating tunneling barrier sandwiched between two magnetic layers. MTJs have been used in magnetic sensors and in the read head for storage media, thus MTJs are key devices in spintronics.1−4 Spintronics researchers are currently seeking a state-of-the-art technology, namely a high-speed and/or highdensity nonvolatile memory, that utilizes a quantum-mechanical current-induced magnetization switching technique in MTJs.5 Memory with ultimately low consumption power can be realized using voltage-controlled magnetic anisotropy (VCMA) as an alternative technique to switch the magnetization.6,7 These intriguing MTJs are currently being developed based on CoFeB ferromagnetic alloys and the MgO insulator, as this material combination exhibits a large tunnel magnetoresistance (TMR) in MTJs.8 Moreover, perpendicular magnetic anisotropy (PMA) emerges from a heterointerface of CoFeB and MgO layers.9 PMA is crucial to provide MTJs with the following functionalities: the protection of the magnetic state from thermal agitation enhanced in nanoscale magnets9 and the VCMA. However, a drawback of the CoFeB alloys is a large magnetization that generates a stray magnetic field nearby, which hinders the high-density integration.10 Ferrimagnets and antiferromagnets are considered as more advanced materials having a small stray field. In particular, films of Mn-based ordered alloys have been intensively studied because of large PMA,11,12 zero-magnetic moment,13 low Gilbert damping,11,14 and high spin-polarization,12,15,16 which have been experimentally demonstrated or theoretically predicted. However, these alloys mostly exhibit the superior © XXXX American Chemical Society

Received: September 8, 2018 Accepted: November 21, 2018

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DOI: 10.1021/acsami.8b15606 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

(2)

R o = (R ap + R p)/2

(3)

Here, Rap and Rp are the resistances for the antiparallel and parallel arrangements of the two magnetizations, respectively. The CoFeB layer has an in-plane magnetic easy axis and an out-of-plane saturation magnetic field of ∼1 T in this study. This is because the CoFeB layer is so thick that the shape magnetic anisotropy overcomes the interfacial PMA of CoFeB/MgO. Thus, the plateau and the minimum of R at μ0H ≈ ±1 T seen in the out-of-plane R−H curve reflect the saturation of the magnetization for the CoFeB layer. Additionally, the gradual change in R between the magnetic field ≈ ±1 T in the out-of-plane R−H curve is attributed to the magnetization rotation of the CoFeB layer from upward to downward as the field decreases, and vice versa. The abrupt changes in R at μ0H ≈ ±1.5 T then correspond to the magnetization switching of the Mn nanolayer, indicating the existence of a perpendicular magnetic easy axis in the Mn nanolayer. The above-mentioned resistance plateau and minimum are attributed to Rp and Rap, resulting in a TMR ratio of −7.3% based on the definition TMR ratio (%) = 100(Rap − Rp)/Rp. Thus, the so-called inverse or negative TMR effect is observed.19−21 Because the CoFeB layer is in-plane magnetized and the Mn nanolayer is out-of-plane magnetized, the in-plane R−H curve corresponds to the magnetization process of the Mn nanolayer, as described in eq 1. The resistance at the zero field for both R−H curves is the same, proving that it corresponds to R0. The gradual change in R in the in-plane R−H curve indicates the very high in-plane saturation field coming from the PMA in the Mn nanolayer. The in-plane R−H curve shows no saturation even at μ0H = 9 T; hence, further high-field in-plane

Figure 1. (a) Junction resistance R for the Mn/MgO/CoFeB MTJ measured at 300 K with application of an out-of-plane and in-plane magnetic field and the bias voltage of −50 mV. (b) High-field in-plane R−H curve for different Mn/MgO/CoFeB MTJs on the same substrate. The junction area for the devices shown in a and b were 20 × 20 and 10 × 10 μm2, respectively. The schematics of the staking structure and the magnetization process for the Mn and CoFeB are shown in the insets of a and b. (c) Out-of-plane magnetization hysteresis for the blanket films without the CoFeB layer.

The junction resistance change due to the TMR effect obeys the following relation: R = R o − ΔRcos θ

ΔR = (R ap − R p)/2

(1)

when |ΔR/R0| ≪ 1. Here, θ is the relative angle of the magnetizations for two magnetic layers, and R0 is the resistance at θ = 90°, i.e., for two orthogonal magnetization directions. Equation 1 also leads to 18

Figure 2. Resistance R and the out-of-plane or in-plane magnetic field H curves measured at 300 K with applied bias voltages of (a) 0.05, (b) 0.3, (c) 0.5, (d) −0.05, (e) −0.3, and (f) −0.5 V, respectively. The device measured is same as the one shown in Figure 1a. (g) Bias voltage dependence of the TMR ratio at 300 K. (h) Normalized MR curves for different bias voltages. B

DOI: 10.1021/acsami.8b15606 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces measurements for different devices on the same substrate were performed up to 19 T, as shown in Figure 1b. Even though the resistance gradually increases, it does not saturate, meaning that the saturation field μ0Hs > 19 T, with a very large magnitude being discovered. Figure 1c shows the out-of-plane magnetization hysteresis curve for the blanket film comprising a Mn nanolayer similarly grown without a CoFeB layer. Although the signal measured is very weak, the perpendicular magnetization at remanance and hysteresis curves are observed, being qualitatively similar to the magnetization process discussed above. The saturation magnetization Ms was estimated as 25 ± 10 kA/m, being negligibly small, even though the TMR effect was definitely observed. The effective field of PMA Heff k and the saturation magnetization are linked by the following relation: μ0 Hkeff = 2K ueff /Ms

Figure 3. Bias voltage V dependence of the PMA energy density Ep evaluated for the junction shown in Figure 1a. The line is fitted to the data.

(4)

assumption of μ0Heff k ≈ μ0Hs > 19 T for the Mn nanolayer. Note that this procedure underestimates the value of Ep, as the in-plane R−H curves were not saturated. Thus, the Ep evaluated here is regarded as the lower bound of the true value of Ep. Hereafter, the atomic structures of and near the Mn nanolayer are analyzed and more details are described in the Supporting Information. Figure 4a shows the cross-section of the MTJ observed via bright-field scanning transmission electron microscopy (STEM). The coherence of the crystalline lattice along the [001] direction from the bottom to the MgO layer is visible. Furthermore, the CoGa layer forms a chemically ordered crystal structure, as observed in the image for the same cross-section measured via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 4b), where the relatively bright or dark spots in the CoGa layer are attributed to the Ga and Co atoms, respectively. The Mn nanolayer seems to be grown on a Ga-terminated CoGa template, which is more visible in the line profile of the STEM intensity taken along the [001] directions for the cross-section, as shown in Figure 4c. Further information regarding the Mn/MgO interface was obtained from the elemental composition profile measured via electron energy-loss spectroscopy (EELS), as shown in Figure 4d. The spectra of O, compared with that of Mg, extends to that of the Mn layer, implying that the Mn nanolayer may be slightly oxidized. From the line profile along the [001] direction and that along the MgO [100] direction (not shown here), the inplane a and out-of-plane c lattice constants, respectively, for each layer were approximately evaluated. The a (c) of the Mn nanolayer is evaluated as 0.29 (0.3) nm when we consider an epitaxial relationship of CoGa(001)⟨100⟩||bct-Mn(001)⟨100⟩. The above-mentioned bct Mn layer in this study resembles a metastable bct Mn grown on an Fe(001) template, as extensively studied in the past.24 For instance, a and c for the bct Mn were, respectively, 0.287 ± 0.002 and 0.304 ± 0.004 nm, as reported by Andrieu et al.25 The bct Mn exhibits a ferromagnetic moment for a thickness of 1−2 monolayers (MLs); it then tends to change their magnetism to layered antiferromagnetic as the thickness increases in the range of 2 to 10 MLs, depending on the growth condition and the presence of capping layers. The behavior can be understood as follows: the two-dimensional form of the bct Mn has a ferromagnetic ground state; then, it tends to have an antiferromagnetic ground state as bulk bct Mn after relaxing the lattice from the epitaxial strain. Yamada et al. confirmed the layered

where Keff u is the effective PMA constant, including an intrinsic constant of PMA Ku and the shape magnetic anisotropy energy density. Thus, the large Hs may reflect a large Keff u and tiny Ms value. Note that these Ms and Hs values are much smaller and higher than the values of 110 kA/m and 16 T observed for the 90 nm thick film of the tetragonal Mn3Ga ferrimagnetic alloy, respectively.16 Figure 2a−f shows the representative R−H curves measured with applying an out-of-plane or in-plane magnetic field and different bias voltages V. The resistance change ΔR obviously decreases as the magnitude of V increases; this trend is basically consistent with those reported for the conventional MTJs. The TMR ratio evaluated at each V is shown in Figure 2g, in which very asymmetric dependence can be observed. Note that the data for the positive bias is plotted only up to 0.3 V, as Rp and Rap were not well-defined at V > 0.3 V due to the change in the shape of the out-of-plane R−H data at approximately 0.5 V, as shown in Figure 2c. The origin of this shape change will be discussed later. Figure 2h displays the in-plane MR curves MR(H) = [R(H) − Rp]/Rp for different negative V that were normalized by the MR value at the zero field. Here, we used the Rp value obtained from the out-ofplane R−H curves at each V. The MR curves clearly indicate the VCMA effect; the bias voltage induced a change in the magnetization process of the Mn nanolayer. The Mn nanolayer becomes easier to magnetize in the plane as the bias voltage V negatively increases thanks to the decrease in Heff k due to the applied voltage. The effect of the current-induced spin-transfertorque may be negligible because a current density used here was smaller than ∼1 MA/m2. Note that the large change in the shape of the out-of-plane R−H data observed for V > 0 in Figure 2c may mostly result from the influence of the tunnel anisotropic magnetoresistance (TAMR) effect and thus the VCMA effect for V > 0 is unable to be discussed here (see the Supporting Information). We obtain insight into the size of the VCMA effect for the Mn nanolayer by following the procedure of the VCMA experiments on MTJs with the conventional materials reported previously (see the Supporting Information).22,23 The PMA energy density Ep was plotted as a function of the bias voltage V in Figure 3. The data exhibit an approximately linear relationship, being fitted by a linear function. The slope of dEp/ dV of 0.020 MJ/Vm3 was evaluated. The Ep at V = 0 V is 0.184 MJ/m3, being close to the lower bound of the Keff u value, i.e., 0.24 MJ/m3, approximately evaluated using eq 4 with the C

DOI: 10.1021/acsami.8b15606 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) TEM and (b) HAADF-STEM image of the cross-section of the junction. (c) Line profile of the STEM intensity, and the schematic of each layer. (d) Line profiles of each element measured by EELS.

antiferromagnetism for the bct Mn on Fe(001),26 while Andrieu et al. observed that the bct Mn showed a ferrimagnetic spin arrangement in the several MLs case before becoming antiferromagnetic;25 this difference probably depends on the surface morphology and defects. The Mn nanolayer in this study is analogously considered since the lattice constant of paramagnetic B2 ordered CoGa is similar to that of Fe. Approximately 4−5 MLs of Mn (001) exist on CoGa (001), as observed in Figure 4c, which may cause the ferrimagnetic spin arrangement to result in finite net magnetization. Some part of the few oxide MLs might be attributed to the NaCl-type MnO, which has an a value of 0.4446 nm in bulk form.27 This MnO ML is considered to inhomogeneously exist together with the MgO barriers on the Mn nanolayer, namely, the Mn nanolayer is inhomogeneously oxidized. The TMR effect is described by Julliere’s model:28 TMR ratio = 2P1P2/ (1−P1P2), where P1,2 is the spin polarization of the electronic density-of-states at the Fermi level for the two metal electrodes. P for CoFeB is known to be positive; thus, P for the interface of the Mn nanolayer is negative to account the negative TMR effect observed in this study. Negative spin polarization has been observed in various MTJs comprising oxide magnets19,20 and antiferromagnetic oxide barriers.21 The above-mentioned MnO nanolayer at the interface may affect the spin polarization of the MTJs in this study. Strong asymmetric bias dependence of the TMR ratio could also stem from the asymmetry of the barrier partially comprising MnO/ MgO, as observed in MTJs with the magnetic and antiferromagnetic oxides.19,21 No reports on the PMA for the bct Mn grown on Fe (001) have been given. The PMA in this study could stem from the interface of the Mn nanolayer and the crystalline oxide of MnO or MgO. This is because the interface PMA may arise as a

result of the Mn−O bonding that was discussed for the Heusler alloy/MgO interface terminated by Mn elements,29 the mechanism being similar to that for the interface PMA arising from the electron orbital hybridization for Fe and O atoms at the Fe/MgO interface.30 The presence of the interfacial PMA is also in accordance with the VCMA effect observed, as the electric field changes the electronic structure and electron population within the screening length, which is typically 1−2 MLs in the case of metals.7 The lower bound of the VCMA coefficient ≡tMntoxide(dEp/dV) in this study was evaluated as ∼35 fJ/Vm for a Mn nanolayer thickness tMn of 0.7 nm and an oxide barrier thickness toxide of 2.5 nm. This lower bound of the VCMA coefficient is a similar order to the lower bound observed for the FeCo/MgO or Fe/MgO system, ∼ 30 to 290 fJ/Vm (see more discussion in Supporting Information).22,23 In summary, we demonstrated MTJs comprising a Mn nanolayer with the perpendicular magnetic easy axis. Althogh the Mn nanolayer showed the negligibly small magnetization, the TMR ratio of −7.3% was observed even at room temperature. The nanolayer also exhibited the large effective PMA field and the reduction of PMA due to the voltage application. The TEM observation showed that a bct Mn-like nanolayer was formed between the CoGa and MgO layers, which exhibited the-above-mentioned extraordinary properties. Although there are many issues, in particular low annealing endurance and oxidation of the Mn layer to be resolved for improving the TMR effect, our findings may provide new insights into a creation of new nanolayer magnets using nonferromagnetic elements. D

DOI: 10.1021/acsami.8b15606 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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(11) Mizukami, S.; Sakuma, A.; Sugihara, A.; Suzuki, K. Z.; Ranjbar, R. Mn-Based Hard Magnets with Small Saturation Magnetization and Low Spin Relaxation for Spintronics. Scr. Mater. 2016, 118, 70−74. (12) Kurt, H.; Baadji, N.; Rode, K.; Venkatesan, M.; Stamenov, P.; Sanvito, S.; Coey, J. M. D. Magnetic and Electronic Properties of D022-Mn3Ge (001) Films. Appl. Phys. Lett. 2012, 101 (13), 132410. (13) Borisov, K.; Betto, D.; Lau, Y.-C.; Fowley, C.; Titova, A.; Thiyagarajah, N.; Atcheson, G.; Lindner, J.; Deac, a. M.; Coey, J. M. D.; et al. Tunnelling Magnetoresistance of the Half-Metallic Compensated Ferrimagnet Mn2RuxGa. Appl. Phys. Lett. 2016, 108 (19), 192407. (14) Mizukami, S.; Wu, F.; Sakuma, A.; Walowski, J.; Watanabe, D.; Kubota, T.; Zhang, X.; Naganuma, H.; Oogane, M.; Ando, Y.; et al. Long-Lived Ultrafast Spin Precession in Manganese Alloys Films with a Large Perpendicular Magnetic Anisotropy. Phys. Rev. Lett. 2011, 106 (11), 117201. (15) Balke, B.; Fecher, G. H.; Winterlik, J.; Felser, C. Mn3Ga, a Compensated Ferrimagnet with High Curie Temperature and Low Magnetic Moment for Spin Torque Transfer Applications. Appl. Phys. Lett. 2007, 90 (15), 152504. (16) Kurt, H.; Rode, K.; Venkatesan, M.; Stamenov, P.; Coey, J. M. D. High Spin Polarization in Epitaxial Films of Ferrimagnetic Mn3Ga. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83 (2), No. 020405(R). (17) Suzuki, K. Z.; Ranjbar, R.; Okabayashi, J.; Miura, Y.; Sugihara, A.; Tsuchiura, H.; Mizukami, S. Perpendicular Magnetic Tunnel Junction with a Strained Mn-Based Nanolayer. Sci. Rep. 2016, 6, 30249. (18) Slonczewski, J. C. Conductance and Exchange Coupling of Two Ferromagnets Separated by a Tunneling Barrier. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 39 (10), 6995−7002. (19) De Teresa; Jose Maria Barthelemy, A.; Fert, A.; Contour; Jean Pierre Montaigne, F.; Seneor, P. Role of Metal-Oxide Interface in Determining the Spin Polarization of Magnetic Tunnel Junctions. Science (Washington, DC, U. S.) 1999, 286 (5439), 507−509. (20) Leo, T.; Kaiser, C.; Yang, H.; Parkin, S. S. P.; Sperlich, M.; Guntherodt, G.; Smith, D. J. Sign of Tunneling Magnetoresistance in CrO2-Based Magnetic Tunnel Junctions. Appl. Phys. Lett. 2007, 91 (25), 252506. (21) Yang, H.; Yang, S.-H.; Qi, D.-C.; Rusydi, A.; Kawai, H.; Saeys, M.; Leo, T.; Smith, D. J.; Parkin, S. S. P. Negative Tunneling Magnetoresistance by Canted Magnetization in MgO/NiO Tunnel Barriers. Phys. Rev. Lett. 2011, 106 (16), 167201. (22) Shiota, Y.; Murakami, S.; Bonell, F.; Nozaki, T.; Shinjo, T.; Suzuki, Y. Quantitative Evaluation of Voltage-Induced Magnetic Anisotropy Change by Magnetoresistance Measurement. Appl. Phys. Express 2011, 4 (4), 043005. (23) Nozaki, T.; Koziol-Rachwal, A.; Skowronski, W.; Zayets, V.; Shiota, Y.; Tamaru, S.; Kubota, H.; Fukushima, A.; Yuasa, S.; Suzuki, Y. Large Voltage-Induced Changes in the Perpendicular Magnetic Anisotropy of an MgO-Based Tunnel Junction with an Ultrathin Fe Layer. Phys. Rev. Appl. 2016, 5 (4), 1−10. (24) Hafner, J.; Spisak, D. Ab Initio Investigation of the Magnetism of Tetragonal Mn: Bulk, Surface, Ultrathin Films, and Multilayers. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72 (14), 144420. (25) Andrieu, S.; Finazzi, M.; Bauer, P.; Fischer, H.; Lefevre, P.; Traverse, A.; Hricovini, K.; Krill, G.; Piecuch, M. Growth, Structure, and Magnetic Properties of Thin Mn Films Epitaxially Grown on (001) Bcc Fe. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57 (3), 1985−1991. (26) Yamada, T. K.; Bischoff, M. M. J.; Heijnen, G. M. M.; Mizoguchi, T.; van Kempen, H. Observation of Spin-Polarized Surface States on Ultrathin Bct Mn(001) Films by Spin-Polarized Scanning Tunneling Spectroscopy. Phys. Rev. Lett. 2003, 90 (5), 056803. (27) Morosin, B. Exchange Striction Effects in MnO and MnS. Phys. Rev. B 1970, 1 (1), 236−243. (28) Julliere, M. Tunneling between Ferromagnetic Films. Phys. Lett. A 1975, 54 (3), 225−226.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15606.

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Experimental procedure, TAMR measurements, polar magneto-optical Kerr effect measurements (p-MOKE), evaluation procedure of VCMA coefficient, detailed analysis of the structures of the junctions, and additional discussion on the PMA and VCMA (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *[email protected]. ORCID

Kazuya Z. Suzuki: 0000-0002-9171-3342 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Y. Kondo and Y. Kiguchi for their technical support in the microfabrication and MR measurements and Y. Miura, J. Okabayashi, and T. Miyazaki for fruitful discussions. This work was partially supported by the ImPACT program, KAKENHI (17K14103), the Asahi Glass Foundation, and the Sasakawa Foundation.

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DOI: 10.1021/acsami.8b15606 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX